Electrooptical system

ABSTRACT

A direct view infrared monitoring system including a storage tube having a storage mesh with a photoconductive storage surface thereon sensitive only to infrared radiation, the tube having a photocathode to flood the photoconductive surface with electrons. The photocathode is made of a material which is sensitive only to ultraviolet radiation and is transparent to the infrared. A shutter is employed to allow illumination of the photoconductive surface with the infrared or to stop such illumination during reading and erasing steps. The shutter is opened for writing. According to one or more embodiments of the invention, a luminescent screen is supplied with an increased number of electrons by the use of a channel type electron multiplier positioned between the storage mesh and the screen.

United States Patent 1191 Reif [ Jan. 8, 1974 ELECTROOPTICAL SYSTEM [75] Inventor: Philip George Reif, Chatsworth,

Calif.

[73] Assignee: International Telephone and Telegraph Corporation, New York, N.Y.

[22] Filed: Nov. 4, 1971 [21] Appl. No.: 195,745

' [52] US. Cl. 250/213 VT, 313/65 A, 315/12 [51] Int. Cl. H0lj 31/50 [58] Field of Search 250/207, 213 R, 213 VT; 313/65 R, 65 A; 315/12 [56] References Cited UNITED STATES PATENTS 2,979,621 4/1961 Heimann 250/213 VT 3,322,999 5/1967 Kazan 250/213 VT 3,327,151 6/1967 Adams 250/213 VT 2,992,358 7/1961 Farnsworth 250/213 VT 3,012,149 12/1961 Heimann 250/213 R 3,201,630 8/1965 Orthuber..... 250/213 VT 2,888,513 5/1959 Melamed 313165 A 3,440,428 4/1969 Kazan 250/213 VT 3,256,435 6/1966 Astheimer 250/213 R 3,204,142 8/1965 De Haan 313/65 A FOREIGN PATENTS OR APPLICATIONS 1,022,274 3/1966 Great Britain 250/213 R Primary Examiner--James W. Lawrence Assistant Examiner-D. C. Nelms Attorney-C. Cornell Remsen, Jr. et al.

[ 5 7] ABSTRACT A direct view infrared monitoring system including a storage tube having a storage mesh with a photoconductive storage surface thereon sensitive only to infrared radiation, the tube having a photocathode to flood the photoconductive surface with electrons. The photocathode is made of a material which is sensitive only to ultraviolet radiation and is transparent to the infrared. A shutter is employed to allow illumination of the photoconductive surface with the infrared or to stop such illumination during reading and erasing steps. The shutter is opened for writing. According to one or more embodiments of the invention, a luminescent screen is supplied with an increased number of electrons by the use of a channel type electron multiplier positioned between the storage mesh and the screen.

26 Claims, 5 Drawing Figures 4/ l POWER suPPu/ l jf PATENIEDJAN 8 1974 POWER SUP/ L V 74 WA VEL E/ve TH (/w/cRo/vs) POWER SUPPL 1 70A POWER sup ELECTROOPTICAL SYSTEM BACKGROUND OF THE INVENTION This invention relates to systems for detecting radiation within a certain spectrum, and more particularly, to an electrooptical system including a direct view storage tube.

The system of the present invention will have a great many applications. It, therefore, is not to be limited to only those specific applications disclosed herein. However, the invention will be found to possess considerable utility in monitoring radiation within a predetermined spectrum and displaying the intensity thereof on a luminescent screen visible to the naked eye.

The usefulness of the invention may be better understood from the following explanation of a prior art problem.

In the past, infrared photographs have been the only means of obtaining visual representations of the intensity of infrared radiation over an entire field of view. The time lapse between the exposing and developing of a photographic film has thus imposed a serious limitation upon its use in this case. For example, the search for an aircraft which has made an emergency landing or which has crash landed can be made by a patrolling aircraft in the daytime. However, at night and in the shadows created by mountains either during the day or at dusk or during the night, with the naked eye it is often impossible to see an aircraft in distress which is on the ground.

SUMMARY OF THE INVENTION In accordance with the system of the present invention, the abovedescribed and other disadvantages of the prior art are overcome by providing a direct view storage tube including a luminescent screen, and means to provide electrons to bombard the screen to cause it to emit light visible to the naked eye. The light intensity represents the intensity of radiation received within a predetermined spectrum over the entire extent of a corresponding field of view.

Thus, if the said predetermined spectrum is the infrared spectrum, a continual infrared examination of a field of view may be made in search ofan aircraft in distress.

The above-described and other advantages of the present invention will be better understood from the following detailed description when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings which are to be regarded as merely illustrative:

FIG. 1 is a side elevational view of the system of the present invention, partly in section;

FIG. 2 is a graph illustrating optical characteristics of certain portions of the system of FIG. 1;

FIG. 3 is a vertical sectional view through an alternative portion of the system shown in FIG. 1;

FIG. 4 is a side elevational view, partly in section, of another embodiment of the invention employing the structure shown in FIG. 3; and

FIG. 5 is a side elevational view, partly in section, of still another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings in FIG. 1, the system of the present invention is indicated at including a storage tube 11 having an evacuated envelope 12 including input and output windows 13 and 14, and a glass cylinder 15.

A photocathode 16 is fixed relative to the envelope 12 in a position contiguous to the right face of window 13, as viewed in FIG. 1.

A luminescent screen 17 is fixed relative to envelope 12 contiguous to the left hand face of window 14, as viewed in FIG. 1. Screen 17 may be entirely conventional. It may be an aluminized phosphor screen.

As will be explained, electrons flow from the left hand side of screen 17 and bombard screen 17. Screen 17 then emits light. Thus, output window 14 is transparent to the light so emitted by screen 17.

A cylinder 18 is fixed relative to envelope 12 but spaced from the window 13.

An infrared filter 19 is fixed relative to cylinder 18 inside thereof. The same is true of a lens 20.

A shutter support 21 is fixed relative to both cylinder 18 and envelope 12. Support 21 carries a shutter 22 which may be opened and closed by operation of control knob 23 mounted on support .21.

An ultraviolet filter 24 is also fixed relative to cylinder 18 and envelope 12.

A flood lamp 25 is fixed relative to envelope 12, and illuminates photocathode 16 by shining light through filter 24. One side of lamp 25 may be connected to ground at 26. The other side of lamp 25 may be connected to ground through a switch 27 and a source of potential 28.

A conductive mesh 29 is fixed inside envelope 12 in a position spaced a short distance from the right hand face of photocathode 16. Mesh 29 has a lead sulfide layer 30 fixed to the left hand side thereof as viewed in FIG. 1. Mesh 29 has holes therethrough. Mesh 29 may be either a woven wire mesh or an electroform mesh. Layer 30 likewise has holes therethrough. The holes in layer 30 lie in registration with the holes in mesh 29.

A channel type electron multiplier is illustrated at 31 in FIG. 1. Multiplier 31 is fixed relative to envelope 12 inside thereof. Multiplier 31 may be entirely conventional, if desired. Multiplier 31 includes an input electrode 32, an output electrode 33 and a glass plate 34. Plate 34 has holes 35 which extend completely therethrough. Thus, the plate 34 has internal surfaces which substantially define the extent of holes 35. The said internal surfaces are made secondary emissive.

Electrodes 32 and 33 may be evaporated on the opposite faces of plate 34, as shown. Thus, input electrode 32 has holes therethrough which lie in registration with plate holes 35. The same is true of holes in output electrode 33.

Leads 36, 37, 38, 39 and 40 are connected from photocathode 16, mesh 29, electrode 32, electrode 33 and screen 17, respectively, through the glass cylinder 15. Tube 11 has a power supply 41 having output leads 42, 43, 44, 45 and 46, and a grounded lead 47. Leads 38, 39 and 40 are connected to leads 44, 45 and 46, respectively. Lead 36 is connected to the pole 48 of a switch 49. Switch 49 has a contact 50 and a contact 51. Switch 49 is a single pole, double throw switch. Leads 37 and 43 are connected to contact 51. Lead 42 is connected to contact 50.

Power supply 41 supplies potentials 5 volts, volts, +300 volts, +1 ,000 volts and +6,000 volts to leads 42, 43, 44, 45 and 46, respectively. All these potentials are with respect to ground, the potential of ground being considered 0 volts.

Cathode 16, by itself, may be an entirely conventional photocathode. It is well known in the art as an S-11 photocathode. It is a cesium antimony photocathode. Accordingly, photocathode 16 is photoemissive when illuminated by ultraviolet radiation. However, photocathode 16 does not emit a substantial number of electrons when illuminated by infrared. On the contrary, photocathode 16 is substantially transparent to infrared. Input window 13 of envelope 12 is thus transparent to both infrared and ultraviolet.

The lead sulfide layer 30 is a photoconductor. However, layer 30 is substantially insensitive to the ultraviolet radiation and is sensitive to the infrared radiation.

In FIG. 2, the sensitivity of the S-1 1 is indicated at 52. The percent transmission of filter 24 is indicated at 53. The sensitivity of layer 30 is indicated at 54. The percent transmission of infrared filter 19 is indicated at 55.

OPERATION Erasing For erasing, switch 49 is actuated to place pole 48 in engagement with contact 50. This places photocathode 16 at a potential of volts. Shutter 22 is then closed. The ultraviolet passing through ultraviolet filter 24 then illuminates photocathode 16 when switch 27 is closed. The illumination of photocathode 16 by the ultraviolet then causes electrons to be liberated from the photocathode 16. These liberated electrons impinge on layer 30 and cause a uniform charge over the complete area thereof facing photocathode 16. Writing After erasing, switch 49 is moved to place pole 48 in engagement with contact 51. This places 0 volts on photocathode 16. Switch 27 is then opened to remove the ultraviolet. Shutter 22 is then opened and an infrared image is focused onto layer 30. Depending upon the intensity of the infrared radiation on each elemental area on layer 30 which faces photocathode 16, more or less of the charge in each elemental area is drained off by the function of layer 30 as a photoconductor. That is, the portion of layer 30 underneath an elemental area thereon increases in conductivity as the radiation on the elemental area increases. The phenomena may be explained simply in anothe way. The infrared image is focused onto the layer 30 and varying amounts of electric charge from elemental area to elemental area is bled off through the photoconductor, which layer 30 is, to the backing screen 29. Reading For reading, switch 49 is again operated to place pole 48 in engagement with contact 50. This maintains photocathode 16 at a potential of 5 volts. Shutter 22 is then closed. Lamp is turned on by closing switch 27. Illumination of photocathode 16, then, by the ultraviolet causes a uniform flood of electrons to be accelerated over the area of photocathode 16 toward layer and mesh 29. Layer 30 and mesh 29 then convert the electron flood to what might be called electron image. This is true because more or less electrons will be admitted to one particular hole in mesh 29 depending upon what charge is left on layer 30 in the immediate vicinity or surrounding the said one hole in mesh 29.

For example, if no infrared radiation illuminated the portion of layer 30 surrounding one hole in mesh 29, the charge on layer 30 thereat might keep all of the electrons from passing through the hole encircled by the said portion of layer 30. Conversely, if sufficient radiation were supplied to an area on layer 30 surrounding a hole in mesh 29, all of the electrons from photocathode 16 might pass through the uninhibited mesh hole. That is, all of the photocathode electrons in that vicinity.

The charge on layer 30 after writing, thus determines how many electrons are allowed to pass through each hole in mesh 29. If the charge on an elemental area of layer 30 on the left side thereof, as viewed in FIG. 1, is relatively high, few flood electrons can pass through the holes of mesh 29. Conversely, if the charge has been drained off, a number of flood electrons may pass through the holes of mesh 29.

The electron charge on layer 30 thus acts in a manner similar to a negatively biased control grid of a triode. If the layer 30 has a considerable electron charge remaining thereon after writing, the electron charge on layer 30 will thus repel the flood electrons and the field therefrom will block, or at least partially block, one or more holes in mesh 29.

Multiplier 31, by itself, operates in the conventional way. That is, primary electrons entering the left ends of holes 35 will bombard the sides thereof. This process will be repeated one or more times depending upon the direction of electron entry. The current output of holes 35 will then be considerably greater than the current input thereto. Since screen 17 is maintained at 6,000 volts and output electrode 33 is maintained at 1,000 volts, a substantial accelerating voltage cuases the electron output of multiplier 31 to create a visible image by bombarding screen 17.

THE ALTERNATIVE EMBODIMENT OF FIGS. 3 AND 4 In FIG. 3, a modified electron multiplier 56 is illustrated which may be employed in lieu of layer 30, mesh 29 and muliplier 31, shown in FIG. 1. Multiplier 56 includes a plate 57 identical to plate 34, and input and output electrodes 58 and 59 which may be identical to electrodes 32 and 33, respectively. However, a layer 60 is fixed relative to electrode 58. Layer 60 may be identical to layer 30, shown in FIG. 1. As before, electrode 58 has holes therethrough which lie in registration with holes 61 in plate 57. The same is true of holes in electrode 59. Layer 60 also has holes, as before. However, the holes in layer 60 lie in registration with the holes in electrode 58.

In accordance with the foregoing, it will be appreciated that mesh 29 and electrode 32 in FIG. 1 may or may not be identical, as desired. However, since mesh 29 may be omitted with the multiplier 56, the work mesh is, therefore, hereby defined for use herein and in the claims to include either mesh 29 or an equivalent thereof, or electrode 32 of an equivalent thereof, or electrode 58 or an equivalent thereof.

Multiplier 56 may be incorporated into a direct view storage tube 62, as shown in FIG. 4. Tube 62 includes an evacuated envelope 63 which may be identical to envelope 12. Tube 62 may also be otherwise identical to tube 1 1 except for the use of multiplier 56, the omission of layer 30, mesh 29 and multiplier 31.

Tube 62 may be employed with all of the structures shown in FIG. 1 outside envelope 12 with the exception of the electrical leads from envelope 12, the switch 49, the power supply 41 and the electrical leads from the power supply 41.

As before, tube 62 includes a photocathode 64 and a luminescent screen 65, photocathode 64 also being made of an S-ll material.

Leads 66, 67, 68 and 69 are connected from photocathode 64, electrode 58, electrode 59 and screen 65, respectively, through envelope 63. A power supply is provided at 70 having output leads 7], 72, 73 and 74, and a grounded lead at 75. A switch 76 has one contact 77 and another contact 78. Switch 76 also has a pole 79. Lead 66 is connected to pole 79. Lead 71 is connected to contact 77. Leads 67 and 72 are connected to contact 78. Leads 68 and 73 are connected together. Leads 69 and 74 are connected together.

Potentials are supplied to the output leads 71, 72, 73 and 74 of power supply 70, which potentials are 5 volts, volts, 700 volts and 5,700 volts, respectively.

The erasing, writing and reading steps for the embodiment of FIGS. 3 and 4 are performed exactly in the same way that they are performed with the system of FIG. 1. Note will be taken that only the fixed potentials of electrode 59 and screen 65 are different from those of the corresponding electrode 33 and screen 17 in FIG. 1.

THE ALTERNATIVE EMBODIMENT OF FIG.

In FIG. 5, a direct view storage tube is indicated at 80 having an evacuated envelope 81 including a glass cylinder 82, an input window 83 and an output window 84. Tube 80 also includes a photocathode 85, a photoconductive layer 86, a mesh 87 and a luminescent screen 88. Although photocathode 85 may be identical to photocathode 16, photocathode 85 may be made of a material including gold, if desired. Tube 80 may thus be identical to tube 11, shown in FIG. 1, without multiplier 31. Thus, the structure shown in FIG. 5 may be employed with the same structures shown in FIG. 1 with which the embodiment of FIGS. 3 and 4 may be employed.

Tube 80 has electrical leads 89, 90 and 91 connected, respectively, from photocathode 85, mesh 87 and screen 88 through cylinder 82 of envelope 81. A power supply 91 is provided having output leads 92, 93, 94, 95 and a grounded lead 96. A single pole, double throw switch 97 is provided with a pole 98, a contact 99 and a contact 100. Another single pole, double throw switch 101 is provided with a contact 102 and a contact 103. Leads 89 and 90 are connected respectively to poles 98 and 104 of switches 97 and 101, respectively. Lead 92 is connected to contact 99. Lead 93 is connected to both of the contacts 100 and 102. Lead 94 is connected to contact 103. Leads 91 and 95 are connected together. Potentials -l0 volts, 0 volts, 2 volts and 5,000 volts are supplied on leads 92, 93, 94 and 95, respectively, from power supply 91.

In the operation of the embodiment of FIG. 5, it is the same as the operation of the system shown in FIG. 1, except that somewhat different voltages are employed. and the potential of mesh 87 is changed. In FIG. 1, note will be taken that mesh 29 is always maintained at ground potential. Erasing and reading are performed with photocathode 85 at --l0 volts in FIG. 5. Thus, to supply -l0 volts to photocathode 85, switch arm 98 must engage switch contact 99. Photocathode is maintained at 0 volts or ground during writing. In order to accomplish this, switch arm 98 is then moved to engagement with contact 100.

For erasing and writing, mesh 87 is maintained at 0 volts or ground potential. This is established by placing switch arm 104 in engagement with switch contact 102. For reading, mesh 87 is maintained at 2 volts. This is achieved by placing switch arm 104 into engagement with contact 103.

From the foregoing, it will be appreciated that al though the system 10 is adapted to be used with radiation in two different spectrums, and these spectrums need not be mutually exclusive, it is an advantage of the present invention that substantially mutually exclusive infrared and ultraviolet radiation spectrums and employed.

Notice will also be taken that preferably photocathode 16 is photoemissive when illuminated with ultraviolet, and layer 30 is substantially insensitive to ultraviolet, absolute compliance with these conditions is not necessary. Similarly, photocathode 16 must be at least somewhat transparent to the infrared, photocathode 16 need not be absolutely insensitive to infrared. That is, it, of course, may be somewhat photoemissive when illuminated with infrared radiation. Layer 30, of course, must be at least somewhat sensitive to infrared radiation.

Although the words infrared" and ultraviolet have been frequently used herein as the only radiation spectrums possible, it is to be understood that all statements made herein are hereby broadened to include radiation in any two suitable spectrums.

Note will be taken that only a few holes have been shown in various structures such as layer 30, mesh 29, electrode 32, plate 34 and electrode 33. Only a few holes have been illustrated for clarity. Many more holes may be provided. This is especially true as regards multiplier 31. Multiplier 31 by no means need have a number of holes 35 equal to the number of holes in mesh 29 or vice versa. In general, a larger number of holes 35 in multiplier 31 may result in better resolution.

The present invention is not limited to many of the structures disclosed herein. For example, any of the embodiments of the invention may be operated continuously without a shutter.

Without departing from the invention, layers 30, 60 and 86 and photocathodes 16, 64 and 85 all may be made of materials different from those disclosed herein for infrared, ultraviolet or any other suitable spectrums.

What is claimed is:

1. In an electrooptical system, the combination comprising: a storage tube including an evacuated envelope having radiation transparent input and output windows, an approximately flat photocathode layer fixed relative to said input window contiguous thereto inside said envelope to receive radiations of a first and a second different predetermined spectrums, said photocathode layer being of a type which is sensitive to and emits electrons when illuminated by said radiation of said first predetermined spectrum, a relatively flat conductive mesh fixed relative to said envelope in a position adjacent to and approximately parallel to but spaced from said photocathode layer to receive said radiations of said first and second spectrums passing through said photocathode, said mesh having holes extending completely therethrough in directions from said photocathode layer toward said mesh to pass electrons therethrough, a photoconductive layer fixed to one side of said mesh, which said one side is approximately parallel to and closest to said photocathode layer, said photoconductive layer having holes therethrough which lie in registration with said mesh holes, and a substantially flat luminescent screen fixed relative to said envelope inside thereof in a position contiguous to said output window and in a position approximately parallel to said photocathode layer to receive electrons from said mesh, said mesh and said photoconductive layer being positioned between said photocathode layer and said screen, all lines approximately perpendicular to said photocathode layer and to said screen at least within a predetermined area on said photocathode layer also lying approximately perpendicular to both of said mesh sides, said photoconductive layer being made of a material which is sensitive to and will change conductivity when illuminated by said radiation of said second pre determined spectrum and is relatively insensitive to said radiation of said first spectrum, said input window and said photocathode layer being relatively insensitive and transparent to said radiation of said second spectrum to pass the same when it is directed toward said input window and said photocathode layer from a position outside said envelope onto said photoconductive layer, said output window being transparent to radiation created by electrons bombarding said screen on the side thereof opposite the side on which said output window is positioned; and flood means fixed relative to said envelope to illuminate said photocathode layer with said radiation of said first spectrum.

2. The invention as defined in claim 1, wherein said first spectrum is in the range of ultra-violet radiation and said second spectrum is in the infrared range, the material of said photoconductive layer being of a type which does not change substantially in conductivity when it is illuminated by radiation falling within said ultra-violet range.

3. The invention as defined in claim 2, wherein, said photocathode layer is made of a material which does not emit a substantial number of electrons when it is illuminated by radiation falling within said infrared range.

4. The invention as defined in claim 1, including means supplying first, second and third stepped potentials to said photocathode layer, conductive mesh and luminescent screen respectively, switching means selectively connecting said photocathode between said first and second potentials, and means for selectively actuating said flood means.

5. The invention as defined in claim 4, including a dielectric plate, said plate being fixed relative to said envelope inside thereof, said plate having input and output surfaces substnatially parallel to each other and to said photocathode layer and screen, said plate having a plurality of holes extending completely therethrough in a direction approximately normal to said input and output surfaces, an output conductive electrode on said output surface and fixed relative to said plate, said output electrode having a plurality of holes extending therethrough in registration with said plate holes, said plate holesbeing substantially defined by internal surfaces of said platewhich are secondary emissive.

6. The invention as defined in claim 5, including an input conductive electrode on said inpout surface and fixed relative to said plate, said input electrode having a plurality of holes extending completely therethrough in registration with said plate holes, said input electrode being spaced from said mesh.

'7. The invention as defined in claim 5, wherein said mesh is fixed relative to said plate on the input surface thereof, said mesh holes lying in registration with said plate holes.

8. The invention as defined in claim 4, including a shutter having open and closed positions and being actuable in said closed position to stop radiation of said second spectrum from entering said input window in one direction, said flood means being actuable to illuminate said photocathode layer when said shutter is in both said open and closed positions.

9. The invention as defined in claim 8, including a first filter outside said envelope and fixed relative thereto in spaced relation to said input window, said first filter having a passband which is said second spectrum, said shutter being adapted to stop radiation which would normally pass through said first filter to said photoconductive layer from reaching said input window, said flood means including a radiation source and a second filter positioned between said source and said input window to filter the radiation output of said source, said second filter having a passband which is said first spectrum.

10. The invention as defined in claim 5, including a shutter having open and closed positions and being actuable in said closed position to stop radiation of said second spectrum from entering said input window in one direction, said flood means being actuable to illuminate said photocathode layer when said shutter is in both said open and closed positions.

11. The invention as defined in claim 10, including at least a first filter outside said envelope and fixed relative thereto in spaced relation to said input window, said first filter having a passband whic is said second spectrum.

12. The invention as defined in claim 11, wherein said shutter is adapted to stop radiation which would normally pass through said first filter to said photoconductive layer from reaching said input window.

13. The invention as defined in claim 12, wherein said flood means includes a radiation source and a second filter positioned between said source and said input window to filter the radiation output of said source, said second filter having a passband which is said first spectrum.

14. The invention as defined in claim 10, wherein said flood means includes a radiation source and a filter positioned between said source and said input window to filter the radiation output of said source, said filter having a passband which is said first spectrum.

15. The invention as defined in claim 14, wherein said shutter is adapted to stop radiation which normally would enter said input window from a direction other than the direction of radiation thereto from said source.

16. The invention as defined in claim 15, including a further filter fixed relative to said envelope in a position to filter radiation entering said input window in said other direction, said further filter havng a passband which is said second spectrum.

17. The invention as defined in claim 6, including a shutter having open and closed positions and being actuable in said closed position to stop radiation of said second spectrum from entering said input window in one direction, said flood means being actuable to illuminate said photocathode layer when said shutter is in both said open and closed positions.

18. The invention as defined in claim 17, including at least a first filter outside said envelope and fixed relative thereto in spaced relation to said input window, said first filter having a passband which is said second spectrum, said shutter being adapted to stop radiation which would normally pass through said first filter to said photoconductive layer from reaching said input window, said flood means including a radiation source.

19. The invention as defined in claim 18, wherein said flood means also includes a second filter positioned between said source and said input window to filter the radiation output of said source, said second filter having a passband which is said first spectrum.

20. The invention as defined in claim 17, wherein said flood means includes a radiation source and a filter positioned between said source and said input window to filter the radiation output of said source, said filter having a passband which is said first spectrum, said shutter being adapted to stop radiation which normally would enter said input window from a direction other than the direction of radiation thereto from said source.

21. The invention as defined in claim 20, including further filter fixed relative to said envelope in a position to filter radiation entering said input window in said other direction, said further filter having a passband which is said second spectrum.

22. The invention as defined in claim 7, including a shutter having open and closed positions and being actuable in said closed position to stop radiation of said second spectrum from entering said input window in one direction, said flood means being actuable to illuminate said photocathode layer when said shutter is in both said open and closed positions.

23. The invention as defined in claim 22, including at least a first filter outside said envelope and fixed relative thereto in spaced relation to said input window, said first filter having a passband which is said second spectrum, said shutter being adapted to stop radiation which would normally pass through said first filter to said photoconductive layer from reaching said input window, said flood means including a radiation source.

24. The invention as defined in claim 23, wherein said flood means also includes a. second filter positioned between said source and said input window to filter the radiation output of said source, said filter having a passband which is said first spectrum.

25. The invention as defined in claim 22, wherein said flood means includes a radiation source and a filter positioned between said source and said input window to filter the radiation output of said source, said filter having a passband which is said first spectrum, said shutter being adapted to stop radiation which normally would enter said input window from a direction other than the direction of radiation thereto from said source.

26. The invention as defined in claim 25, including a further filter fixed relative to said envelope in a position to filter radiation entering said input window in said other direction, said further filter having a passband which is said second spectrum. 

1. In an electrooptical system, the combination comprising: a storage tube including an evacuated envelope having radiation transparent input and output windows, an approximately flat photocathode layer fixed relative to said input window contiguous thereto inside said envelope to receive radiations of a first and a second different predetermined spectrums, said photocathode layer being of a type which is sensitive to and emits electrons when illuminated by said radiation of said first predetermined spectrum, a relatively flat conductive mesh fixed relative to said envelope in a position adjacent to and approximately parallel to but spaced from said photocathode layer to receive said radiations of said first and second spectrums passing through said photocathode, said mesh having holes extending completely therethrough in directions from said photocathode layer toward said mesh to pass electrons therethrough, a photoconductive layer fixed to one side of said mesh, which said one side is approximately parallel to and closest to said photocathode layer, said photoconductive layer having holes therethrough which lie in registration with said mesh holes, and a substantially flat luminescent screen fixed relative to said envelope inside thereof in a position contiguous to said output window and in a position approximately parallel to said photocathode layer to receive electrons from said mesh, said mesh and said photoconductive layer being positioned between said photocathode layer and said screen, all lines approximately perpendicular to said photocathode layer and to said screen at least within a predetermined area on said photocathode layer also lying approximately perpendicular to both of said mesh sides, said photoconductive layer being made of a material which is sensitive to and will change conductivity when illuminated by said radiation of said second predetermined spectrum and is relatively insensitive to said radiation of said first spectrum, said input window and said photocathode layer being relatively insensitive and transparent to said radiation of said second spectrum to pass the same when it is directed toward said input window and said photocathode layer from a position outside said envelope onto said photoconductive layer, said output window being transparent to radiation created by electrons bombarding said screen on the side thereof opposite the side on which said output window is positioned; and flood means fixed relative to said envelope to illuminate said photocathode layer with said radiation of said first spectrum.
 2. The invention as defined in claim 1, wherein said first spectrum is in the range of ultra-violet radiation and said second spectrum is in the infrared range, the material of said photoconductive layer being of a type which does not change substantially in conductivity when it is illuminated by radiation falling within said ultra-violet range.
 3. The invention as defined in claim 2, wherein, said photocathode layer is made of a material which does not emit a substantial number of electrons when it is illuminated by radiation falling within said infrared range.
 4. The invention as defined in claim 1, including means supplying first, second and third stepped potentials to said photocathode layer, conDuctive mesh and luminescent screen respectively, switching means selectively connecting said photocathode between said first and second potentials, and means for selectively actuating said flood means.
 5. The invention as defined in claim 4, including a dielectric plate, said plate being fixed relative to said envelope inside thereof, said plate having input and output surfaces substnatially parallel to each other and to said photocathode layer and screen, said plate having a plurality of holes extending completely therethrough in a direction approximately normal to said input and output surfaces, an output conductive electrode on said output surface and fixed relative to said plate, said output electrode having a plurality of holes extending therethrough in registration with said plate holes, said plate holes being substantially defined by internal surfaces of said plate which are secondary emissive.
 6. The invention as defined in claim 5, including an input conductive electrode on said inpout surface and fixed relative to said plate, said input electrode having a plurality of holes extending completely therethrough in registration with said plate holes, said input electrode being spaced from said mesh.
 7. The invention as defined in claim 5, wherein said mesh is fixed relative to said plate on the input surface thereof, said mesh holes lying in registration with said plate holes.
 8. The invention as defined in claim 4, including a shutter having open and closed positions and being actuable in said closed position to stop radiation of said second spectrum from entering said input window in one direction, said flood means being actuable to illuminate said photocathode layer when said shutter is in both said open and closed positions.
 9. The invention as defined in claim 8, including a first filter outside said envelope and fixed relative thereto in spaced relation to said input window, said first filter having a passband which is said second spectrum, said shutter being adapted to stop radiation which would normally pass through said first filter to said photoconductive layer from reaching said input window, said flood means including a radiation source and a second filter positioned between said source and said input window to filter the radiation output of said source, said second filter having a passband which is said first spectrum.
 10. The invention as defined in claim 5, including a shutter having open and closed positions and being actuable in said closed position to stop radiation of said second spectrum from entering said input window in one direction, said flood means being actuable to illuminate said photocathode layer when said shutter is in both said open and closed positions.
 11. The invention as defined in claim 10, including at least a first filter outside said envelope and fixed relative thereto in spaced relation to said input window, said first filter having a passband whic is said second spectrum.
 12. The invention as defined in claim 11, wherein said shutter is adapted to stop radiation which would normally pass through said first filter to said photoconductive layer from reaching said input window.
 13. The invention as defined in claim 12, wherein said flood means includes a radiation source and a second filter positioned between said source and said input window to filter the radiation output of said source, said second filter having a passband which is said first spectrum.
 14. The invention as defined in claim 10, wherein said flood means includes a radiation source and a filter positioned between said source and said input window to filter the radiation output of said source, said filter having a passband which is said first spectrum.
 15. The invention as defined in claim 14, wherein said shutter is adapted to stop radiation which normally would enter said input window from a direction other than the direction of radiation thereto from said source.
 16. The invention as defined in claim 15, including a further filtEr fixed relative to said envelope in a position to filter radiation entering said input window in said other direction, said further filter havng a passband which is said second spectrum.
 17. The invention as defined in claim 6, including a shutter having open and closed positions and being actuable in said closed position to stop radiation of said second spectrum from entering said input window in one direction, said flood means being actuable to illuminate said photocathode layer when said shutter is in both said open and closed positions.
 18. The invention as defined in claim 17, including at least a first filter outside said envelope and fixed relative thereto in spaced relation to said input window, said first filter having a passband which is said second spectrum, said shutter being adapted to stop radiation which would normally pass through said first filter to said photoconductive layer from reaching said input window, said flood means including a radiation source.
 19. The invention as defined in claim 18, wherein said flood means also includes a second filter positioned between said source and said input window to filter the radiation output of said source, said second filter having a passband which is said first spectrum.
 20. The invention as defined in claim 17, wherein said flood means includes a radiation source and a filter positioned between said source and said input window to filter the radiation output of said source, said filter having a passband which is said first spectrum, said shutter being adapted to stop radiation which normally would enter said input window from a direction other than the direction of radiation thereto from said source.
 21. The invention as defined in claim 20, including further filter fixed relative to said envelope in a position to filter radiation entering said input window in said other direction, said further filter having a passband which is said second spectrum.
 22. The invention as defined in claim 7, including a shutter having open and closed positions and being actuable in said closed position to stop radiation of said second spectrum from entering said input window in one direction, said flood means being actuable to illuminate said photocathode layer when said shutter is in both said open and closed positions.
 23. The invention as defined in claim 22, including at least a first filter outside said envelope and fixed relative thereto in spaced relation to said input window, said first filter having a passband which is said second spectrum, said shutter being adapted to stop radiation which would normally pass through said first filter to said photoconductive layer from reaching said input window, said flood means including a radiation source.
 24. The invention as defined in claim 23, wherein said flood means also includes a second filter positioned between said source and said input window to filter the radiation output of said source, said filter having a passband which is said first spectrum.
 25. The invention as defined in claim 22, wherein said flood means includes a radiation source and a filter positioned between said source and said input window to filter the radiation output of said source, said filter having a passband which is said first spectrum, said shutter being adapted to stop radiation which normally would enter said input window from a direction other than the direction of radiation thereto from said source.
 26. The invention as defined in claim 25, including a further filter fixed relative to said envelope in a position to filter radiation entering said input window in said other direction, said further filter having a passband which is said second spectrum. 